As explained above, the means by which frequency resources are utilised in conventional OFDMA-MIMO schemes prevents or significantly limits interference among users within a given cell. In other words, intra-cell interference is substantially avoided. However, in the more elaborate multi-cellular networks discussed in the previous paragraph, the benefits of OFDMA and MIMO transmission can often be limited by inter-cell interference.
Inter-cell interference may arise, for example, because the frequency resources (i.e. the carriers and subcarriers) utilised by base stations in transmitting data to users in one cell are identical to the frequency resources utilised by base stations in transmitting data to users in an adjacent cell. In other words, in the kinds of wireless communication systems in which the present invention may find use, there is likely to be, using terminology common in the art, 1:1 frequency reuse between adjacent cells. The effect of this can be particularly significant for so-called “cell-edge users” located near the boundary between cells. For a cell-edge user, the distance to the one base station currently serving that user may be roughly the same as, or only marginally different to, the distances to the base stations that are in adjacent cells. As a result, from the point of view of the user near the cell edge, the signal strength received from the serving base station may be only marginally stronger than, or approximately the same as, the signal strength from the base stations in the adjacent cells, as seen by the cell-edge user. And because common frequency resources are used in adjacent cells (i.e. there is simultaneous use of substantially identical transmission frequencies in adjacent cells), signals being transmitted in the adjacent cells can often interfere with data being transmitted to the cell-edge user.
One method which has been proposed for addressing this difficulty is to coordinate the MIMO transmissions among multiple base stations (i.e. coordinating transmissions in adjacent or nearby cells) to eliminate or reduce this inter-cell interference. A full explanation of the techniques employed to achieve this coordination is not necessary for the purposes of this explanation. For present purposes it is sufficient to note that this coordination can reduce or eliminate inter-cell interference among coordinated cells (or coordinated portions of cells) and this can result in a significant improvement in the coverage of high data rates, cell-edge throughput and/or overall system throughput. However, the trade-off for this improvement is that the coordination of transmissions in multi-cellular MIMO systems requires channel state information (CSI) and data information to be shared among the coordinated base stations. This in turn results in a significant additional burden on the system's transmission and data capacity resources. In particular, for FDD systems, base station channel knowledge is mainly obtained by user equipment (UE) feedback. Since multiple cells (or multiple sectors of cells) participate in the coordinated transmission, the amount of channel knowledge required to be fed back increases linearly with the number of cooperating cells (or the number of cooperating cell sectors). It will be appreciated that this can place a heavy burden on the uplink channel particularly.
As explained in the previous paragraph, coordinated multi-cell MIMO transmission/reception (also often referred to as coordinated multi-point transmission/reception or CoMP) may be used to improve the coverage of high data rates, cell-edge throughput and/or to increase system throughput. The downlink schemes used in CoMP may be considered to fall into the following two categories:                “Coordinated Scheduling and/or Coordinated Beamforming (CS/CB)” and        “Joint Processing/Joint Transmission (JP/JT)”.        
Incidentally, those skilled in the art will be generally familiar with the basics and underlying principles of beamforming, which is a signal processing technique that makes use of constructive and destructive interference to assist with directional signal transmission and/or reception. Further explanation of beamforming is therefore not required here.
In CS/CB, data to a single UE is instantaneously transmitted from one transmission point, but decisions regarding user scheduling (i.e. the scheduling of timings for transmissions to respective users) and/or beamforming decisions are made with coordination among the cooperating cells (or cell sectors). In other words, scheduling/beamforming decisions are made with coordination between the cells (or cell sectors) participating in the coordinated scheme.
On the other hand, in JP/JT, data to a single UE is simultaneously transmitted from multiple transmission points to (coherently or non-coherently) improve the received signal quality and/or cancel interference for other UEs.
FIG. 6 schematically illustrates the working principles of the two above-mentioned categories of downlink transmission used in CoMP, although it should be noted that the way the base stations are illustrated relative to the distribution of the cells in FIG. 6 may not accurately reflect the true distribution of base stations vis-à-vis cells in practical wireless communication system implementations. Nevertheless, FIG. 6 is sufficient for present purposes to illustrate the principles of CS/CB and JP downlink transmission schemes respectively, used in CoMP.
Joint Processing (JP) is represented in FIG. 6(a) in which cells A, B and C actively transmit to the UE, while cell D is not transmitting during the transmission interval used by cells A, B and C.
Coordinated scheduling and/or coordinated beamforming (CS/CB) is represented in FIG. 6(b) where only cell B actively transmits data to the UE, while the user scheduling/beamforming decisions are made with coordination among cells A, B, C and D so that the co-channel inter-cell interference among the cooperating cells can be reduced or eliminated.
In the operation of CoMP, UEs feed back channel state information. The channel state information is often detailed, and often includes measurements of one or more of channel state/statistical information, narrow band Signal to Interference plus Noise Ratio (SINR), etc. The channel state information may also include measurements relating to channel spatial structure and other channel-related parameters.
As explained above, feedback of channel state information allows modification of the transmitted signal (typically modification by the base station(s) prior to transmission) to account for changing channel conditions and to maximise data throughput. More specifically, it is often done in order to perform precoder design, link adaptation and scheduling at the base stations. As also explained above, for FDD systems, the amount of channel information needed to be fed back increases linearly with the number of cooperating cells (or sectors of cells), and this creates a heavy additional burden for the uplink channel particularly.
Furthermore, high performance CoMP transmission schemes are often very sensitive to channel information being outdated. Therefore, the benefits of CoMP may diminish, or may not be as realisable, in high mobility scenarios such as, for example, where UE velocities are 120 km/h or higher, since the feedback requirements necessary to enable CoMP to function optimally or well for such high velocities are difficult to satisfy. It is therefore envisaged that the present invention will typically be implemented in the context of CoMP in low and medium mobility scenarios such as, for example, with UE velocities up to approximately 30 km/h. However, it should be clearly understood that no strict limitation is to be implied from this, and the invention might also be implemented in higher mobility scenarios, possibly despite the challenges posed by the feedback requirements, or perhaps if means are devised for minimising or streamlining etc the feedback requirements making them easier to satisfy.
The remainder of this section discusses previously proposed methods and other disclosures related to the feedback of channel state information.
Chinese Patent Application No. CN101415229 discusses a method for feeding back channel state information (CSI) based on a limited feedback limit in a space division multiple access system. The method uses a quantization error threshold and a channel gain threshold to select users that should feed back CSI.
International Patent Application No. PCT/US2008/056579 presents a system and methodology for compressing CQI (channel quality indices/indicators) feedback at the receiver to reduce redundancy in CQI feedback information in wireless communications systems. To compress the CQI in the frequency domain, the main idea in this document is to feed back the CQIs of subbands whose CQIs have changed the most; and to compress the CQI in the time domain. The discrete cosine transform is performed on the CQI.
In Taiwanese patent TW274482, a method that can be applied in MIMO-OFDM systems is disclosed to reduce the feedback rate. The main proposal is to feed back the CSI for a part of frequency bands, and then CSI of all frequency bands can be represented by an interpolation on the feedback.
U.S. Pat. No. 7,359,470 proposes a method to determine the minimum feedback rate for CSI in MIMO systems. A method for estimating an expected performance loss in capacity based on the most recent feedback of the CSI is proposed. The method estimates expected performance loss based on estimated spatial covariance information. By using the estimation on the expected performance loss, the maximum tolerable channel feedback delay is calculated with which the expected capacity is greater than a predetermined threshold, and the minimum feedback rate is derived based on the maximum tolerable channel feedback delay.
In European Patent No. 1437854, the variable channel quality feedback rate in a wireless communication system is proposed based on the presence or absence of a transmission from the base station to the mobile station.
U.S. Pat. No. 7,050,759 discloses a method that reduces feedback overhead by using three separate feedback subchannels. The main idea is that the information carried on each subchannel can be used separately or together by a base station to selectively update internal registers storing channel conditions.